1. Field of the Invention
The present invention relates to the compensation method for waveform degradation due to wavelength dispersion in an optical transmission system, and in particular, it relates to the wavelength chirping of an optical transmitting device.
2. Description of the Related Art
Since a transmission waveform degrades due to wavelength dispersion in a large-capacity and long-haul optical communication system (optical transmission system with a transfer rate of 10 Gb/s or more), dispersion compensation is indispensable. Regarding the dispersion compensation methods, there are the following three main methods. By optimizing each of the parameters, long-haul transmission is realized.
FIG. 1 shows the outline of a long-haul WDM system and how to compensate for dispersion.
In FIG. 1, first, the electrical/optical (E/O) conversion unit 10 of an optical transmitting unit 10 generates optical signals with a variety of frequencies. For example, E/O1, E/O2, . . . , E/On generate an optical signal with wavelength λ1, an optical signal with wavelength λ2, . . . , and an optical signal with wavelength λn, respectively. An optical multiplexer 11 multiplexes optical signals with respective wavelengths that are generated in this way and inputs the multiplexed optical signal to a transmitting compensation fiber 12 as a wavelength division multiplexing (WDM) signal. Then, an optical transmitting amplifier 13 amplifies the WDM signal and transmits it to a transmission fiber 14. In the receiver, firstly, a receiving amplifier 15 amplifies the WDM signal having been propagated through the transmission fiber 14, a receiving dispersion compensation fiber 16 then compensates for dispersion and an optical demultiplexer 17 demultiplexes the WDM signal into optical signals with respective wavelengths. Then, the respective optical/electrical converters O/E 1 through O/E n of a receiving unit 18 converts each optical signal with its respective wavelength into its respective electrical signal, and the electrical signals are processed.
In the configuration described above, the following methods are adopted as a method for compensating for degradation due to dispersion of an optical signal:    (1) A method for optimizing the wavelength chirping of an optical transmitting unit    (2) A method for inserting a dispersion compensation fiber that negates the amount of wavelength dispersion experienced over a transmission line    (3) A method for increasing optical transmitting power and using self-phase modulation (SPM)
As the means for realizing (1), a configuration using a Mach-Zehnder optical modulator is well known.
FIGS. 2 through 5 show the basic configuration of a Mach-Zehnder optical modulator and its operation.
In the Mach-Zehnder optical modulator shown in FIG. 2, an optical input is demultiplexed into two inputs carried by arms 1 and 2. The respective arms are connected to electrodes 1 and 2, as shown, and voltages V1 and V2 are applied to the electrodes 2 and 1, respectively. In this case, capacitor C1 is inserted between a power source V1 and the electrode 2 and capacitor C2 is inserted between a power source V2 and the electrode 1 to eliminate respective DC biases. When a voltage is applied, the light phase f the input signal changes. Therefore, when an optical input with its original phase is demultiplexed and combined again, the optical input is reproduced with a different phase. This means that if the input optical signal is multiplexed with a different phase, then the output optical signal increases or decreases depending on the magnitude of the phase difference. In this way, optical intensity modulation can be realized.
FIG. 3 shows the relationship between the optical output of the Mach-Zehnder optical modulator and an applied voltage.
As shown in FIG. 3, the intensity of an optical output periodically varies with the change of an applied voltage. Here, the magnitude of an applied voltage is V0, when the optical output obtained is lowest and the magnitude of applied voltage is Vπ, when the optical output is highest. For example, when the voltage of V1 is changed and the voltage of power source V2 is 0 or is grounded, the optical output is lowest if the voltage of the power source V1 is 0 or V0. When the voltage of V1 is Vπ, the optical output is highest. In this case, the optical output changes as shown in (1) of FIG. 3.
However, when V1 is 0 or is grounded and V2 changes, the optical output changes as shown in (2) of FIG. 3. In this case, V2 changes from the lowest to the highest in the range from 0 or V0 to Vπ. However, since the voltage polarity applied to the Mach-Zehnder optical modulator in the case of (1) and that in the case of (2) are opposite, the direction of change of the optical output in the case of (1) and that in the case of (2) are also opposite.
If voltages V1 and V2, each with the same value and opposite polarity, are applied to each of the power sources V1 and V2, respectively, the horizontal axis of FIG. 3 indicates the voltage difference between voltages V1 and V2.
FIG. 4 shows the relationship between the driving voltage amplitude and the optical output of the Mach-Zehnder optical modulator. If the driving voltage amplitude changes in the range from 0 to Vπ, an optical waveform output is obtained, as shown in FIG. 4, due to the relationship between the optical output and the driving voltage of the Mach-Zehnder optical modulator.
The wavelength chirping control method of the Mach-Zehnder optical modulator is realized by applying driving signals V1 and V2 to two branched arms (optical waveguides) and maintaining an amplitude ratio between the two driving signals constant. A specific example of a driving voltage setting method for wavelength chirping is described below. If the Mach-Zehnder optical modulator is driven at a voltage amplitude ratio of 1:0 between respective driving voltages applied to the two driving signals (in other words, a driving signal is input only to one arm), the amount of chirping becomes 1. If the Mach-Zehnder optical modulator is driven while respective voltage amplitudes of the two driving signals are the same, the amount of chirping becomes 0.
FIG. 5 shows the relationship between the driving voltage amplitudes V1 and V2 of the Mach-Zehnder optical modulator, and a chirping coefficient α.
FIG. 5 shows a case where the sum of the respective absolute amplitude voltages of V1 and V2 becomes Vπ. 
In FIG. 5, if V1 and V2 are Vπ, and 0, respectively, the chirping coefficient α becomes −1. If V1 is gradually reduced and V2 is gradually increased, the chirping coefficient α gradually increases. When V1 and V2 are 0 and Vπ, respectively, the chirping coefficient α becomes 1.
In this way, in the Mach-Zehnder optical modulator, the chirping coefficient given to an optical signal after modulation varies depending on the voltage applied to each of the two arms.
The chirping coefficient α is generally given according to the following equation:α=(dφ/dt)(2dP/dt)  (1)
In the above equation, φ and P are optical phase and optical intensity, respectively.
FIG. 6 shows an optical output waveform and a wavelength time response characteristic corresponding to a chirping coefficient.
It is assumed that an optical signal with a waveform shown at the top of FIG. 6 is output from the Mach-Zehnder optical modulator. In this case, in FIG. 6, the chirping coefficient α defined by the above equation is indicated by dotted lines. The chirping coefficient α plays a role when an optical output waveform ascends and when it descends. Specifically, since optical intensity does not change for the duration where the optical intensity of an optical output waveform is constant, the value of the chirping coefficient a becomes 0, as seen from equation (1). Therefore, the chirping coefficient α is not defined over this duration.
If as in FIG. 2, V1 and V2 are Vπ and 0, respectively, the amount of chirping changes as follows. If an optical output waveform ascends, the optical wavelength is deviated upwardly. If the optical output waveform descends, the wavelength is deviated downwardly. In such a case, the chirping coefficient α is positive. In FIG. 6, for such a case as this, α=+1. If each of V1 and V2 is Vπ/2, there is no deviation of an optical wavelength even when the optical output waveform ascends or descends, and the chirping coefficient α becomes 0. If V1 and V2 are 0 and Vπ, respectively, the chirping coefficient α becomes −1. In such a case, the optical wavelength deviates downwardly when the optical output waveform ascends, and it deviates upwardly when the optical output waveform descends.
Although in equation (1), it seems to be the amount of change of a phase that is most important, in reality, it is in fact the time taken for a change of a phase to occur that is important. Therefore, the influence of chirping is observed as a wavelength deviation. Specifically, if light is expressed as follows,P∝cos2(ωt+φ)  (2)Frequency is defined as the time differential of a phase as follows.Frequency=ω+dφ/dt (strictly, an angular frequency is described)  (3)
If the wavelength of light, the speed of light and the refractive index of a medium propagated by light are λ, c and n, respectively, the following equation holds true.ωλ=2πc/n  (4)Then,λ=2πc/(nω)  (5)If this ω is substituted into equation (3), the following equation is obtained.λ=2πc/(n(ω+dφ/dt))  (6)This indicates that the time deviation of a phase is observed as a wavelength deviation.
In a long-haul WDM system, the amount of transmission line wavelength dispersion varies depending on the wavelength. Therefore, the optimum settings value of the chirping coefficient α varies for each wavelength. Accordingly, the ability to allow flexible chirping settings for an optical transmitter is required (for example, the change of α in the range from −1 to +1 must be measured in steps of 0.1).
Since the chirping of an optical transmitter using a Mach-Zehnder optical modulator is set by the ratio between voltages applied to each of the two arms, a method for monitoring driving voltage amplitude is effective. However, the method has the following problems:    (1) If the optical phase modulation efficiency of the driving voltage varies, the prediction of the amount of chirping by the measurement of the driving voltage leads to the occurrence of an error in the chirping coefficient that is wrongly observed as the correct chirping coefficient, due to the fluctuation of the driving voltage in each arm of the Mach-Zehnder optical modulator.    (2) Similarly, there is a difference between a monitored chirping coefficient and an actual chirping coefficient, due to a variety of electrode lengths in each arm of the Mach-Zehnder optical modulator.    (3) There is a high possibility that there are positive and negative chirpings in the vicinity of zero chirping (V1=V2=V90 /2) due to an amplitude detection error, and a transmission waveform degrades.
In the conventional optical transmitter using such a Mach-Zehnder optical modulator, the amplitude of each driving signal must be optimally adjusted in order to optimally control the amount of chirping. However, it is difficult to flexibly set chirping.